This invention relates to a wire electrode pull-up mechanism in a wire-cut electric discharge machine and, more particularly, to a wire electrode pull-up mechanism for gripping and pulling up a wire electrode.
A so-called wire-cut electric discharge machine, namely an electric discharge machine which uses a wire electrode as a cutter, operates by impressing a pulsed voltage between a wire electrode and a workpiece to produce an electrical discharge or spark across the intervening space for the purpose of eroding the workpiece where the spark occurs. By simultaneously moving the workpiece relative to the wire electrode on the basis of machining command data, the workpiece may be cut into any desired contour. There are occasions where the wire electrode breaks within the workpiece during the cutting operation if such machining cutting conditions as the wire electrode feed speed and tension are improper. In the event of such breakage, it is required that this fact be sensed by circuitry within the discharge machine and that the wire electrode be restored automatically without delay.
An apparatus for restoring a wire electrode has already been proposed by the Inventors in, for example, Japanese Patent Application No. 54-075227. With this previously disclosed apparatus, a limit switch for sensing breakage of the wire electrode is operable to suspend an electric discharge cutting operation immediately upon sensing breakage of the electrode during the passage thereof through the workpiece. At the same time, a clamping mechanism located below the workpiece is activated to clamp the wire electrode and prevent it from falling on the supply side. The used wire electrode, namely the wire electrode passed through the workpiece and located on the take-up side, is then pulled up by feed rollers and fed into a wire electrode disposal mechanism where the wire may be cut up into small pieces, by way of example. Next, the remaining portion of the wire electrode which has broken off in the workpiece is pulled downwardly until the upper end of the broken wire is extracted from the workpiece. Thereafter, with the aid of a guiding device such as a nozzle, said end of the wire electrode is again fed into the slit formed in the workpiece by the electric discharge until the wire end emerges a predetermined distance from the top side of the workpiece. Now, a wire electrode pull-up mechanism located on the take-up side of the workpiece grips the protruding end of the wire electrode, pulls the wire electrode upwardly and feeds it into the feed rollers which then grasp the electrode between them, thereby completing the restoration of the wire electrode.
In the prior-art apparatus described above, the wire electrode pull-up mechanism is provided with a wire electrode gripping device having a construction which will now be described with reference to FIGS. 1(a), 1(b) and 1(c).
FIG. 1(a) is a perspective view and FIGS. 1(b) and 1(c) are sectional views illustrating the gripping device of the conventional wire electrode pull-up mechanism. In FIGS. 1(a) and 1(b), the gripping device has an arm AR at the distal end formed to include a downwardly tapered hole BH and a downwardly flared guide hole CH communicating therewith. The tapered hole BH receives a rotatable clamping roller DB movable freely up and down within the hole. In restoring a wire electrode 2 which has broken, the arm AR is lowered until it abuts against the top surface of the workpiece 12, as shown in FIG. 1(b). This is followed by passing the end of the broken wire electrode 2 upwardly through the workpiece 12 and into the guiding hole CH and tapered hole BH of the arm AR until the wire end pushes up the clamping roller DB and emerges from the arm on the upper side thereof, as illustrated in FIG. 1(c). Thenceforth, the arm AR is raised in the direction of the arrow shown in FIG. 1(c), whereupon the clamping roller DB slides downwardly under its own weight along the wall of the tapered hole BH. Owing to the oblique wall of the hole BH, the roller DB wedges the wire electrode 2 against the opposing surface to clamp the wire in place. Raising the arm AR therefore causes the wire electrode 2 to be pulled upwardly as the arm ascends.
A problem encountered in a wire-cut electric discharge machine having a gripping device of the above-described type involves the matter of wire diameter. Specifically, for reasons of cutting precision and the like, it is preferred to make use of a wire electrode which is as slender as possible. However, owing to the construction of the gripping device which requires that the end of the wire electrode push up the clamping roller, the wire must have sufficient rigidity and, hence, there is a limitation on how slender the wire may be. Accordingly, there is need of a wire electrode pull-up mechanism capable of clamping and pulling up even a very slender wire electrode.